1. Field of the Invention
The present invention relates to a method for producing a toner used in a developer for developing a latent electrostatic image in, for example, electrophotography, electrostatic recording and electrostatic printing; to a toner produced with the production method; and to a developer containing the toner.
2. Description of the Related Art
Developers used conventionally in, for example, electrophotography, electrostatic recording and electrostatic printing adhere, in a developing step, to an image bearing member (e.g., a latent electrostatic image bearing member) on which a latent electrostatic image has been formed; then, in a transfer step, are transferred from the image bearing member onto a recording medium (e.g., recording paper sheet); and then, in a fixing step, are fixed on the surface of the recording medium. As have been known, such developers that develop a latent electrostatic image formed on the image bearing member are roughly divided into two-component developers formed of a carrier and a toner and one-component developers requiring no carrier (magnetic or non-magnetic toners).
As the above-described toner, a pulverized toner is widely used, which is produced by melt-kneading a toner binder (e.g. a styrene resin and a polyester resin) together with a colorant, followed by finely pulverizing.
Also, the recent interest has focused on polymerization toners produced with the suspension polymerization method or the emulsion polymerization aggregation method. In addition, Japanese Patent Application Laid-Open (JP-A) No. 07-152202 discloses a polymer dissolution suspension method. In this method, toner materials are dispersed and/or dissolved in a volatile solvent such as an organic solvent having a low boiling point; and the resultant liquid is emulsified in an aqueous medium in the presence of a dispersant to form liquid droplets; and the volatile solvent is removed from the liquid droplets while shrinking the volume thereof. Unlike the suspension polymerization method and the emulsion polymerization aggregation method, the polymer dissolution suspension method is advantageous in that a wider variety of resins can be used; in particular, a polyester resin can be used which is used for forming a full-color image having transparency and smoothness in image portions after fixing.
The polymerization toners must be prepared in an aqueous medium in the presence of a dispersant and thus, the dispersant remains on the surface of the formed toner particles and degrades chargeability and environmental stability thereof. In order to avoid such an unfavorable phenomenon, the remaining dispersant must be removed using a very large amount of wash water and thus, the production method for the polymerization toner is not necessarily satisfactory.
As a method for producing a toner replacing the above polymerization toner, for example, JP-A No. 2003-262976 discloses a method in which a toner composition liquid is formed into microdroplets by piezoelectric pulsation, and the thus-formed microdroplets are solidified through drying to produce toner particles. Also, JP-A No. 2003-280236 discloses a method in which a toner composition liquid is formed into microdroplets by the action of thermal expansion, and the thus-formed microdroplets are solidified through drying to produce toner particles. In addition, JP-A No. 2003-262977 discloses a method in which a toner composition liquid is formed into microdroplets using an acoustic lens, and the thus-formed microdroplets are solidified through drying to produce toner particles.
However, these methods pose a problem in that the number of liquid droplets that can be ejected from one nozzle per unit of time is limited to make their productivity low. Furthermore, it is difficult to prevent the particle size distribution of the formed toner from broadening due to aggregation of liquid droplets. Thus, these methods are far from satisfaction in terms of monodispersibility of the formed toner as well as productivity.
Also, JP-A Nos. 2006-28432 and 2006-28433 disclose a method in which toner materials containing a thermosetting resin or UV curable resin is finely dispersed in a dispersion medium; the resultant dispersion is intermittently discharged from nozzles in the form of liquid droplet; the formed liquid droplets are aggregated and then a thermosetting resin or UV curable resin is cured for stabilizing particle formation. This method, however, exhibits low productivity and forms toner particles having insufficient monodispersibity, similar to the above-described methods disclosed in JP-A Nos. 07-152202, 2003-262976, 2003-280236 and 2003-262977. The toner produced with this method does not have a sufficient fixing property, although the resin is cured after toner particle formation.
The above granulation method disclosed in JP-A Nos. 2006-28432 or 2006-28433 is characterized in that an excitation part (vibration part) is in direct contact with a fluid. In this configuration, when the number of the excitation part is identical to that of micropores (orifices) (i.e., excitation parts correspond one-to-one to micropores (orifices)), the formed toner have a sharp particle size distribution. Meanwhile, when a plurality of micropores and one excitation part are used, the size of liquid droplets discharged from micropores varies with the distance between the excitation part and each micropore and thus, toner particles formed from liquid droplets discharged from different micropores (orifices) have different particle diameters.
For producing a high-quality image, toners have been improved by, for example, making the toner particle diameter smaller lo or the particle size distribution narrower. The toner particles produced with the common kneading pulverizing toner production method have an amorphous shape and thus, are further pulverized through stirring together with carrier particles in the development area of an image forming apparatus. In addition, when used as a one-component developer, the above toner particles are further pulverized through contact with, for example, a developing roller, a toner-feeding roller, a layer thickness-controlling blade and a frictionally charging blade. As a result, extremely fine particles are formed and a flowability improver is embedded in the toner surface, resulting in degrading image quality. Also, the toner particles having such a shape exhibit poor powder flowability and thus, require a large amount of a flowability improver. Furthermore, the filling rate of a toner bottle with such toner particles becomes low, preventing downsizing of apparatuses.
Also, transfer processes for forming a full-color image become more complicated, which transfer multi-color toner images from photoconductors onto a recording medium or paper. When the pulverized toner having an amorphous shape is used in the transfer processes, print through is often observed on the formed image due to its poor transferability and a large amount of toner must be consumed for compensating the print through, which is problematic.
Under such circumstances, there are increasing needs to more reliably transfer toner particles, to reduce the amount of toner consumed, to form high-quality image involving no image through, and to reduce running cost. When transfer efficiency is very high, there is not required to be provided a cleaning unit for removing toner particles remaining the photoconductor or transfer medium. Other advantageous effects are as follows: apparatuses can be downsized, cost reduction can be attained, and no toner to be disposed of is generated. In order to overcome the above-described problems caused by toner particles having an amorphous shape, attempts have been made to develop various production methods for spherical toner particles.
For example, JP-A Nos. 2000-75549 and 2001-249485 disclose toner particles containing, in combination, a styrene resin and a polyester resin excellent in low-temperature fixing property. However, these toner particles, which are produced with the kneading pulverizing method in which a toner composition is melt-kneaded, finely pulverized and classified, have variation in their shape and surface structure. These shape and surface structure slightly vary depending on pulverization property of materials used and on the conditions for a pulverization step, and cannot be easily controlled as desired. Also, a toner having a narrower particle size distribution is difficult to produce in consideration of cost elevation and the limit of classification ability. In the case of pulverized toners, it is very important that their average particle diameter calculated from the particle size distribution thereof is small (in particular, 6 μm or smaller) in consideration of production yield, productivity and cost.
Meanwhile, spherical toner particles having a smaller particle diameter can be easily produced with a toner production method in which a toner composition is discharged from nozzles having small pore size, but nozzle clogging problematically arises in this method. Particularly when a toner containing a releasing agent (wax) is produced, coarse or aggregated wax particles in a toner composition easily cause nozzle clogging and thus, it is essential that the particle diameter of dispersed wax particles is desirably controlled.
In view of the above, demand has arisen for a toner production method in which a toner composition liquid is discharged from fine nozzles to form toner particles and which can efficiently produce a toner having a small particle diameter with very reduced fine powder; and for a toner, as produced with the toner production method, which causes no filming on a photoconductor, etc., is excellent in offset resistance and low-temperature fixing property, has a monodisperse particle size distribution which has not been attained with a conventional method, has very small variation in many characteristic values (e.g., flowability and chargeability), and can form a high-resolution, high-definition, high-quality image involving no degradation in image quality for a long period of time.